This invention relates to a flat sealing gasket, particularly a cylinder head gasket, made of impregnated asbestos fiber and including an armor which has a U-shaped cross section and which frames edge portions of the gasket, particularly for the purpose of surrounding a circular gasket opening intended to be in registry with a combustion chamber of an internal combustion engine.
Cylinder head gaskets intended for internal combustion engines are preferably made of sheet metal (steel) or an impregnated asbestos fiber material reinforced with metal components. Such a head gasket is provided with a plurality of relatively large circular openings for alignment with the combustion chambers and is further provided with relatively smaller openings intended to be aligned with ports through which lubricating and/or coolant flows or through which securing bolts pass. Particularly the openings intended for the combustion chambers and occasionally also the openings to be aligned with ports carrying liquid are surrounded (framed) by an armor formed of a cross-sectionally U-shaped metal frame which, with the legs of the "U", straddles edge portions of the gasket. Such armor is provided for increasing the sealing pressure and the sealing pressure force in the gasket zones sandwiched between the legs of the armor. For increasing the micro-seal or macro-seal and for the particular protection against the effects of the fluids, the entire outer surface of the armor is provided with a synthetic layer.
During the operation of an internal combustion engine the cylinder head gasket is, in addition to the static stress in an axial direction, further stressed by shifts of the engine block and the cylinder head in the radial direction. As described in an article entitled "Der `Nahmaschineneffekt` an Flachdichtungen" ("The `Sewing Machine` Effect in Flat Gaskets") published by the Reinz Company in its publication entitled "Informationen aus der Dichtungstechnik", Issue 10, 1975, page 32, such displacements are caused principally by the alternating thermal expansion and contraction of the engine block and cylinder head during the alternating heating and cooling of the engine during operation. Particularly in case of engines having aluminum cylinder heads and cast iron engine blocks, relative displacements between these two components occur because of the different coefficients of thermal expansion of the two metals. This results in a frictional shearing stress exerted on the cylinder gasket, adversely affecting the sealing properties thereof.
In cylinder head gaskets the above-noted armor is also exposed to stresses. In particular, in the zones of curvatures at the cut edges of the gasket opening, breaks may occur which may progressively lead to the destruction of the entire armor. As disclosed, for example, in German Pat. No. 1,650,026, it has been the view heretofore that only axially oriented oscillations between the engine block and the cylinder head stress the curved portions of the armor and cause breakages thereof. It has therefore been attempted to render the armor particularly resistant against bending by inserting thereinto spring rings and/or by particularly shaping the curved portions. It has been found however, that breakages could not be reliably eliminated by these measures.
In order to avoid wear of sealing faces of cylinder head gaskets, caused by the above-discussed shifting motions of the engine block and the cylinder head gasket, it is known, as disclosed, for example, in U.S. Pat. No. 4,103,913, to coat the sealing faces of metallic seals--which do not have armors around the openings--with friction reducing coatings, essentially consisting of fluoropolymers having molybdenum disulfide additives. By reducing the sliding friction in this manner, a force transmission to the sealing faces is essentially eliminated and thus a destruction of the cylinder head gasket is prevented. The use of such gaskets, however, because of the low-friction coating, significantly increases the amplitude of the shifting motions of the engine block and the cylinder head. Such substantially greater displacements disadvantageously reduce the effect of the micro-seal and the macro-seal of the gasket, particularly as concerns the hot combustion gases.